I. Field
The disclosed embodiments relate to the field of voltage controlled oscillators.
II. Background
A cellular telephone or other wireless communication device transmits and receives signals at specific frequencies. One or more voltage controlled oscillators, commonly referred to as VCOs, are typically used to set or establish desired transmit and/or receive frequencies. One basic type of VCO design is that of an inductor-capacitor (LC) resonant tank VCO.
In an LC resonant tank VCO, one common way by which the frequency is set, entails utilizing a set of metal-insulator-metal capacitors (MIMcaps) which can be switched on and off. By selectively switching MIMcaps, the center frequency of the VCO can be coarse tuned. Furthermore, MIMcaps confer improved compensation for process variation in the fabrication of VCOs. MIMcaps also provide a wider VCO frequency tuning range than would typically be available when a VCO is only implemented with traditional voltage variable capacitors (i.e., varactors). Moreover, using MIMcaps enables the VCO to have a lower tuning sensitivity, also called Kv, than is typically available when a VCO is only implemented with traditional varactors because the varactor elements can be smaller for the same VCO frequency tuning range.
When utilizing MIMcaps for tuning the frequency of an LC resonant tank VCO, there are typically two different MIMcaps configurations. One common configuration entails implementing the LC resonant tank VCO with a set of binary weighted MIMcaps. Specifically, the center frequency of the VCO can be coarse tuned or adjusted over a relatively wide range of frequencies simply by selectively controlling the binary weighting of the MIMcaps. Although this binary weighting scheme offers flexibility, versatility, adaptability, and scalability, it suffers in that it commonly results in suboptimal coverage over the entire VCO tuning frequency range. For example, with a binary weighted VCO implemented with a varactor (which is common practice), there are typically either gaps in the tuning frequency range where the varactor must cover a wider range of frequencies than desired or, conversely, there are overlaps whereby adjacent digital coarse frequency tuning settings are crowded too close together and the varactor is underutilized. In many instances, a binary weighted VCO exhibits both undesired gaps as well as overlaps across its respective frequency range. This disadvantage is virtually impossible to eliminate, given the realistic analog parasitics associated with the binary weighted scheme.
The other common MIMcaps configuration for tuning the frequency of an LC resonant tank VCO entails implementing the VCO with a thermometer coded MIMcaps tuning bank. Specifically, the center frequency of the VCO can be coarse tuned over a relatively wide range of frequencies simply by activating a specific number of MIMcap units, wherein each unit includes a similar amount of capacitance. Consequently, by using the thermometer coded scheme in VCOs, a more optimal spacing of digital coarse tuning in frequency can be achieved. However, when extended to cover larger numbers of bits to support a wider frequency range, a thermometer coded MIMcap tuning bank requires a relatively large section of silicon. In other words, the thermometer coded MIMcap tuning bank may grow to consume a large area of a chip's limited silicon die area. This is highly disadvantageous because either the chip must be made larger or other functionalities must be compromised. In addition, the unavoidable parasitics resulting from the larger and larger biasing circuitry associated with implementing smaller thermometer MIMcap units would also grow correspondingly. Furthermore, the smaller thermometer MIMcap units also tend to result in net worse Q factor; the Q factor represents the quality factor of the LC resonant tank of the VCO. A lower Q factor directly translates into a degradation of the VCO phase noise and power consumption.